What are the root causes of World War I? What led Europe at the height of its power to plunge into such a self-destructive conflict? Two historians, Gerd Krumeich of Duesseldorf University and John Horne of Trinity College, Dublin, offer their explanations.

GERD KRUMEICH: "Without a doubt, the roots of the conflict lie with the rivalries born of European nations' imperial ambitions. At the turn of the century, all believed that having an empire was vital for their development, even their survival in a world faced with rapid industrialization and international competition.

Germany — at that point Europe's leading industrial power — was keen to acquire a colonial empire to match its economic dynamism. But it set about doing so in an aggressive way that was to upset the balance of powers on the continent. To Britain's alarm, Berlin embarked upon an naval arms race, it squabbled with France over African territories, and helped the Ottoman Empire — Russia's great rival — to modernize its army.

These initiatives were countered by Germany's rivals, leaving it with a sense of frustrated ambition. Berlin felt encircled by the British, French and Russians — who themselves felt threatened by Germany's ambitions and closed ranks against it. That state of affairs fueled an arms race during 1912 and 1913, coupled with a flare-up of nationalist sentiment in both Germany and France.

In Berlin, military leaders believed a war was coming and thought they could win only if it came soon — before Russia was able to finish reinforcing its army. That explains the key role played by Germany in triggering the conflict."

JOHN HORNE: "For decades before 1914, an ideological rivalry set the dynastic and multi-ethnic empires of Eastern Europe against the principle of nationality, incarnated by the nation-states of Western Europe and founded on the principle of popular sovereignty.

In the Balkans, emergent nationalism, especially that of Serbia, threatened Austria-Hungary in particular. At the same time, the balance of power in Europe was profoundly modified by the unification of Germany in 1871.

This turned Germany into a great power while French strength gradually declined. Colonial and economic rivalries exacerbated these tensions, but were not their main cause.

The balance of power gradually came to depend on the equilibrium between two armed alliances and on concerted action by the great powers on both sides to prevent regional crises inflaming the entire continent.

This mechanism worked to limit the two Balkan Wars in 1912-13. But in July 1914 it failed. If those in power had understood the nature of the war to come, they would certainly not have embarked on it so casually.

Instead, they considered war a rational option, a risk certainly, but not one that would transform the very nature of the world in which they lived."